用全局变量接收创建的Image对象，防止临时变量被回收而导致统计数据丢失

log.js

// http://oldj.net/article/one-thing-to-notice-about-new-image/
var img = new Image();
var rnd_id = "_img_" + Math.random();
 
window[rnd_id] = img; // 全局变量引用
 
img.onload = img.onerror = function () {
	window[rnd_id] = null; // 删除全局变量引用
}
 
img.src = "http://log.mysite.com/1.gif?a=1&b=2&c=xxx";